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In my organic chemistry lab, we recently had an experiment where we were given a mixture of 50:50 ethanol & ethylene glycol. We were supposed to perform a simple distillation to calculate the percent recovery. Based on my readings, I found that the ethanol has a lower boiling point than ethylene glycol and concluded that ethanol would be the compound that would vaporize and transfer over to the cooled flask where it could condense back. At the end of the lab, we just weighed the distillate and the holdup.

My question was, how do you know if the simple distillation process was successful? I assumed that it was successful based on the fact that it produced a distillate, but I realized that can't be the only reason. The difference in boiling point was large enough to be performed as a simple distillation, but how can I conclude that it was overall successful?

Also, based on this, can I figure out if there is an azeotrope for the boiling points of the two compounds? I read that azeotropes "prefer" to be boiled into the gas phase together, but doesn't that imply that both compounds could have condensed, making the simple distillation process unsuccessful?

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  • $\begingroup$ By definition an azetrope is a distillate that boils at a constant temperature, but is a mixture of two or more compounds. So you can't purify an azetrope by using more distillations. $\endgroup$
    – MaxW
    Nov 7, 2015 at 8:04
  • $\begingroup$ Pure ethanol has a bp of 78.37 °C and pure ethylene glycol has a bp of 197.3 °C. The bp of the azetrope could be (1) below pp of ethanol, or (2) above bp of ethylene glycol. In theory the azetrope cannot be between or equal to either bp. The problem with theory is experimental error. $\endgroup$
    – MaxW
    Nov 7, 2015 at 8:18
  • $\begingroup$ @MaxW I'm not sure if it's an error on my part, the mixture began to produce the distillate around 67°C and began boiling around that time, too. Since that was lower than the bp of ethanol, would the mixture still be considered as an azetrope? Thank you! $\endgroup$
    – lalalalalla
    Nov 7, 2015 at 21:02
  • $\begingroup$ Were you measuring the temperature in the liquid or in the gas phase? I didn't find any evidence of an azetrope between ethanol and ethylene glycol with a google search. I'd suspect that you were measuring the temperature in the distillate (in the gas phase). That is tricky since the thermometer has some thermal mass. It takes a little while for the gaseous distillate to heat it up. $\endgroup$
    – MaxW
    Nov 7, 2015 at 21:16
  • $\begingroup$ @MaxW The thermometer was like this: docbrown.info/page12/gifs/distill.gif, so I assumed that it was measuring the temperature of distillate when it vaporized. I think it makes sense that that they are azeotropes based on what I found on Google, too, but I just wasn't sure how to back up that observation. $\endgroup$
    – lalalalalla
    Nov 7, 2015 at 21:24

1 Answer 1

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By definition an azetrope is a distillate that boils at a constant temperature, but is a mixture of two or more compounds. So you can't purify an azetrope by using more distillations.

Pure ethanol has a bp of 78.37 °C and pure ethylene glycol has a bp of 197.3 °C. The bp of the azetrope could be (1) below pp of ethanol, or (2) above bp of ethylene glycol. In theory the azetrope cannot be between or equal to either of the boiling points of the two pure chemicals. (The problem with theory is experimental error....)

I can find no evidence of an azetrope between ethanol and ethylene glycol using Google. So I very confident that no such azetrope exists.

You noted that the distillate temperature was about 67 °C as the pot started to boil. You also noted that you read the thermometer when it began to produce the drops of distillate.

Look at the drawing below.

glass distillation setup

The glassware setup has two thermometers.

  • Thermometer-1 is in the pot.
  • Thermometer-2 is in the pot.

Ideally the pot temperature and the distillate temperature would be the same. However when the solution starts to boil the distillate is just starting to contact the thermometer. The thermometers have some thermal mass, so it takes time to get them up to temperature.

The distillate thermometer is relatively slow to equilibrate. So long as it is dripping distillate solution then it is colder than the distillate and will not give an accurate temperature reading.

So the only explanation for this anomalous result is that you made a small mistake taking the reading.

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